Nearly thirty years ago it became apparent that household laundry detergents made of branched alkylbenzene sulfonates were gradually polluting rivers and lakes. Solution of the problem led to the manufacture of detergents made of linear alkylbenzene sulfonates (LABS), which were found to biodegrade more rapidly than the branched variety. Today, detergents made of LABS are manufactured world-wide.
LABS are manufactured from linear alkyl benzenes (LAB). The petrochemical industry produces LAB by dehydrogenating linear paraffins to linear olefins and then alkylating benzene with the linear olefins in the presence of HF. This is the industry's standard process. Over the last decade, environmental concerns over HF have increased, leading to a search for substitute processes employing catalysts other than HF that are equivalent or superior to the standard process. Four of the chief criteria for a substitute process are: extent of conversion, selectivity to monoalkylbenzene, linearity of alkylbenzene, and catalyst deactivation rate. At this point, it is useful for the sake of clarity later to define these terms.
i. Alkylation conversion
In alkylation benzene typically is supplied in excess, and therefore conversion is defined in terms of the olefin. The degree of conversion at a constant ratio of excess benzene relative to olefin and a constant temperature is a measure of a catalyst's activity in a process. The degree of conversion may be expressed by the formula, EQU V=C/T*100,
where V equals percent conversion, C equals moles of olefin consumed, and T equals moles of olefin initially present.
ii. Alkylation selectivity
Selectivity is defined as the percentage of total olefin consumed under reaction conditions which appears as monoalkylbenzene and can be expressed by the equation, EQU S=M/C*100,
where S equals percent selectivity, M equals moles of monoalkylbenzenes produced, and C equals moles olefin consumed. The higher the selectivity, the more desirable the process. An approximate measure of selectivity is given by the equation, EQU S=(weight monoalkylbenzene)/(weight total products)*100
where "total products" includes monoalkylbenzenes, polyalkylbenzenes, and olefin oligomers. At high selectivity (S.ltoreq.85%) the results calculated from the two equations are nearly identical. The latter of the foregoing two equations is routinely used in commercial practice because of the difficulty in distinguishing between oligomers and polyalkylbenzenes.
iii. Linearity
The reaction of linear olefins with benzene in principal proceeds according to the equation, EQU C.sub.6 H.sub.6 +R.sub.1 CH=CHR.sub.2 .fwdarw.C.sub.6 H.sub.5 CH(R.sub.1)CH.sub.2 R.sub.2 +C.sub.6 H.sub.5 CH(R.sub.2)CH.sub.2 R.sub.1.
Note that the side chain is branched solely at the benzylic carbon and contains only one branch in the chain. Although strictly speaking this is not a linear alkylbenzene, the terminology which has grown up around the process and product in fact includes as linear alkylbenzenes those materials whose alkyl group chemically arises directly from linear olefins and therefore includes alpha-branched alkylbenzenes. Because alkylation catalysts also may induce the rearrangement of olefins to give products which are not readily biodegradable, for example, alpha, alpha-disubstituted olefins which subsequently react with benzene to afford an alkyl benzene with branching at other than the benzylic carbon, ##STR1## the degree to which the catalyst effects formation of linear alkyl benzenes is another important catalyst parameter. The degree of linearity can be expressed by the equation, EQU D=L/M*100,
where D equals degree of linearity, L equals weight of linear monoalkyl benzene produced, and M equals weight of monoalkyl benzene produced.
iv. Catalyst Deactivation Rate
Alkylation processes, with either HF or substitute catalysts for HF, are subject to catalyst deactivation. Whereas an alkylation process employing HF typically employs an HF regenerator, an alkylation process employing a substitute catalyst such as a solid alkylation catalyst typically includes means for periodically taking the catalyst out of service and regenerating it by removing the gum-type polymers that accumulate on the surface of the catalyst and block reaction sites. For a solid alkylation catalyst, therefore, the catalyst life is measured in terms of time in service at constant conversion between regenerations. The longer the time between regenerations, the more desirable the catalyst and the process.
Of these criteria, the linearity criterion is assuming added importance and significance in view of the expectation in some areas of minimum standards for linearity in detergents of 92-95% near-term, increasing to 95-98% by about the year 2000. Our solution to the problem of identifying a process for detergent alkylation which meets the increasingly stringent requirements of linearity began with our observation that the isomerization of linear olefins to non-linear olefins--this is the process ultimately responsible for non-linear detergent alkylate arising from a linear olefin feedstock--is quite sensitive to temperature but relatively insensitive to the particular catalyst for the detergent alkylate process. This result suggested that effecting alkylation at a lower temperature was the key to greater product linearity. Our focus then shifted to finding a substitute process with a catalyst other than HF that would catalyze detergent alkylation at lower temperatures. Paradoxically, alkylation processes that employ catalysts other than HF, that is, commercially available solid alkylation catalysts, tend to operate at higher alkylation temperatures than processes that employ HF. One reason is that solid alkylation catalysts tend to deactivate faster as alkylation temperatures decrease and therefore alkylation temperatures must be increased to meet the catalyst life requirement. A second reason is that commercially available solid alkylation catalysts are less active than HF and therefore alkylation temperatures must be increased to meet the conversion requirement.
Surprisingly, our solution to this dilemma of alkylation temperature is a new process that arose from our observations of paraffin dehydrogenation. At this point, it is useful for the sake of appreciating what follows to define two terms related to paraffin dehydrogenation.
i. Dehydrogenation Conversion
The degree of conversion at constant temperature is a measure of the catalyst's activity in a dehydrogenation process. The dehydrogenation conversion may be expressed by the formula, EQU Y=U/P*100,
where Y equals percent conversion, U equals weight of linear paraffin consumed, and P equals weight of linear paraffin initially present.
ii. Dehydrogenation Selectivity
However high the conversion may be, a dehydrogenation process is not valuable unless it is also selective. The dehydrogenation of linear paraffins in principal proceeds according to the equation, EQU R.sub.1 CH.sub.2 --CH.sub.2 R.sub.2 .fwdarw.R.sub.1 CH.dbd.CHR.sub.2 +H.sub.2.
The double bond of the product monoolefins is distributed along the chain in substantially equilibrium proportions. Because dehydrogenation catalysts also may induce further dehydrogenation as well as rearrangement to give aromatics which subsequently are alkylated by monoolefins to afford undesirable alkyl aromatic by-products with more than one alkyl group, ##STR2## and other heavier alkyl aromatic by-products with more than one aromatic ring, the selectivity to which the catalyst effects formation of linear monoolefins is another important process parameter. Selectivity is defined as the percentage of total linear paraffin consumed under reaction conditions which appears as linear monoolefin and can be expressed by the equation, EQU W=O/U*100,
where W equals percent selectivity, O equals weight of linear monoolefin produced, and U equals weight of linear paraffin consumed. The better the selectivity, the better is the process.
Our solution to identifying a process for detergent alkylation at lower alkylation temperature arose from our observation that the life of catalyst in the alkylation process is quite sensitive to dehydrogenation selectivity but relatively insensitive to the particular catalyst for the alkylation process. Our observation suggested that reducing the aromatic by-products from dehydrogenation was the key to longer catalyst life in the alkylation process. Our focus then shifted to finding a process that would reduce the aromatic by-products in the dehydrogenation product.
The importance of our observation that the aromatic by-products from dehydrogenation is a major factor in alkylation catalyst life and that the particular catalyst plays only a minor role cannot be overemphasized, for it permits one to focus solely on methods of reducing the aromatic by-products. It is well known that these aromatic by-products are formed during paraffin dehydrogenation at the conversions typical of commercial applications, and so their presence has been long recognized. But they were believed to have no effect other than on dehydrogenation selectivity and alkylation selectivity, which were not considered sufficient to justify the cost of removing the aromatic by-products. We have found that the actual effect of the aromatic by-products is much greater, because reducing them significantly lengthens the life of the solid alkylation catalyst, without the need to raise the alkylation temperature, thereby permitting LAB production at lower alkylation temperatures which in turn produces higher linearity. A result of our observation is a novel process that uses an aromatics removal zone to permit dehydrogenation at typically high conversions and to permit alkylation at substantially lower temperature than that previously attainable by the prior art process.
LAB processes are described in the book edited by R. A. Meyers entitled "Handbook of Petroleum Refining Processes" (McGraw Hill, N.Y. 1986) and "Ullmann's Encyclopedia of Industrial Chemistry," Volumes A8 and A13, Fifth Edition (VCH, Weinheim, Germany). Flow schemes are illustrated in U.S. Pat. No. 3,484,498 issued to R. C. Berg, U.S. Pat. No. 3,494,971 issued to E. R. Fenske, U.S. Pat. No. 4,523,048 issued to Vora which teaches use of a selective diolefin hydrogenation zone, and U.S. Pat. No. 5,012,021 issued to B. Vora which teaches use of a selective monoolefin hydrogenation zone. Solid alkylation catalysts are illustrated in U.S. Pat. No. 3,201,487 issued to S. Kovach et al.; U.S. Pat. No. 4,358,628 issued to L. Slaugh; U.S. Pat. No. 4,489,213 issued to S. Kovach; and U.S. Pat. No. 4,673,679 issued to D. Farcasiu. Zeolitic solid alkylation catalysts are disclosed in U.S. Pat. Nos. 3,751,506; 4,387,259; and 4,409,412.
It is well known that polynuclear aromatic compounds and aromatic compounds may be selectively removed from a hydrocarbon processing stream on suitably selected sorbents including alumina, silica gel, cellulose acetate, synthetic magnesium silicate, macroporous magnesium silicate, macroporous polystyrene gel and graphitized carbon black. All of the above-mentioned sorbents are mentioned in a book authored by Milton L. Lee et al entitled "Analytical Chemistry of Polycyclic Aromatic Compounds" and published by Academic Press, N.Y. in 1981. A wide variety of means are disclosed for removing polynuclear aromatic compounds and aromatic compounds from a hydrocarbon stream. U.S. Pat. No. 4,447,315 issued to Lamb et al. teaches a hydrocracking process wherein polynuclear aromatics are adsorbed from a liquid stream. U.S. Pat. No. 2,395,491 issued to Mavity discloses a process for removing mononuclear from polynuclear aromatic compounds. U.S. Pat. No. 2,983,668 issued to Hemminger teaches use of a molecular sieve or silica gel to remove aromatics or n-paraffins from a reaction mix. U.S. Pat. No. 3,340,316 issued to Wackher et al. and U.S. Pat. No. 3,689,404 issued to Hofer et al. teach removal of hydrocarbons using activated carbon.
All of the above references are silent about the effect of aromatic by-products formed during paraffin dehydrogenation on the activity and lifetime of catalysts, especially solid alkylation catalysts, used in LAB processes.